Lymphocyte colonies result when lymphoid cells are exposed to mitogeneic or antigenic stimulation and cultured in semisolid medium. Depending upon the specific system, they are recognized as discoid clusters of cells occuring at the medium-air interface and/or as foci of more diffusely organized cells occuring within the medium. It is not clear whether colonies of differing morphology arise by similar or different mechanisms. Two hypotheses can be envisioned for the generation of lymphoid colonies. The first postulates that lypmhoid colonies arise from clonal expansion of a single precursor cell, while the second envisions that they represet polyclonal associations of cells that have coalesced as a result of mitogen induced chemotacic signals. Though grossly similar in appearance, colonies generated by these two mechanisms would mark the occurance of events having very different biological significance. Clonal mechanisms of lymphocyte colony formation may be viewed as a model for maturation of cells of the lymphoid series, or expansion of mature committed lymphocytes, depending upon the developmental level at which clonal expansion occurs. Alternatively, colonies formed by the influx of lymphoid cells toward a chemotactic signal, may serve as an appropriate model for the recruitment and organization of mononuclear cells that occurs in vivo at a localized site of antigen presentation (e.g. delayed hypersensitivity reaction). In the present proposal experiments designed to examine the mechanism(s) of lymphocyte colony formation with special reference to these hypotheses are presented. Broadly, they include i) the characterization of the cell population(s) responsible for colony formation by selection of cell subsets on the basis of phenotypic markers and functional and physical properties. Depletion - replacement and kinetic studies will be conducted in order to detect possible interactions among subsets, ii) the characterization of cells present in lymphocyte colonies with respect to homogeneity/.heterogeneity including experiments designed to identify proliferating cells within colonies, and to determine whether non-proliferating cells are incorporated into colonies. Both mitogen and antigen induced colonies will be evaluated as it is possible that, depending upon the inducing agent or other less obvious variables, either or both of the above mechanisms may be operative.